The present invention relates to improved thermite coated molten metal filters and the method of making them. It also relates to aqueous thermite coating slurries applied to molten metal filters, to improve the performance, and increase the durability of such filters, in particular, in applications involving molten steel.
In processing molten metals, reduced metals are heated to temperatures above their melting points and then poured into castings either for purposes of storage or for molding. Typically, the molten metal is poured through a ceramic filter to entrap any inclusions which may be present in the molten metal. In most cases, the molten metal temperature is at levels approaching the functional limits of the ceramic filters. In addition, the filter is subject to chemical corrosion which may lead to structural weakening of the filter. Most of the inclusions filtered out of the molten metal are oxide based impurities, and it has been found that the filter efficiency is optimal when the surface energy of the ceramic filter matches, as close as possible, that of the oxide based impurities.
It has also been found that some of the problems encountered in filtering molten metals are caused by the freezing of the metal as it contacts the filter. Since the filter is at a temperature significantly less than the molten metal temperature, the initial molten metal which contacts the filter must impart heat to the filter. Since the filter draws heat from the metal, the part of the molten metal in contact with the filter, decreases in temperature sometimes causing the metal to freeze in the filter. As the metal solidifies in the filter, the solid metal blocks the filter and inhibits the filter performance.
Another problem commonly encountered with molten metal filters, particularly filters formed with aqueous based slurries, is the tendency of the alloys of Fe/Al and Cr/Al, to react vigorously with water to generate hydrogen gas (bubbles) in the slurry solution. The hydrogen evolution continues over long periods of time leading to a steady increase in the pH, and degradation of the slurry with time.
A more severe problem encountered with molten metal filters is "gassing" which is a function of the type and amount of organic compounds present in a thermite coating slurry. During the filtering process and while the molten metal is being poured through a coated filter, organic ingredients in the coating tend to burn off rapidly to generate gas which rushes through the filter and molten metal pour. The presence of such gaseous impurities affects the quality of the final cast product after the molten metal is solidified. If such gassing is too severe, flying metal pieces will appear and the coating may crack or separate from the substrate. In more severe cases, the filter may split apart resulting in a catastrophic failure. The above problems of hydrogen formation and gassing remain major concerns with aqueous coating formulations.
Several methods have been suggested for removing gaseous impurities. One such method is degassing, a process which involves injecting a fluxing gas into the molten metal. There are several major drawbacks associated with such methods including the need to provide large multiple chamber arrangements necessary to sufficiently treat large quantities of molten metal batches. In addition, large pressure drops have been observed in using such systems.
Few devices and methods have been suggested for dealing with the many problems associated with molten metal filters. One improved molten metal filter which addresses many of the above problems including the problems associated with freezing is disclosed in U.S. Pat. No. 5,022,991, titled THERMITE COATED FILTERS, issued to Day et al., assigned to the assignee of the present invention, and herein incorporated by reference. The reference discloses the application of a thermite coating onto a molten metal filter surface to avoid freezing during the filtering process.
While the thermite coated molten metal filter of Day et al., offers significant improvements over the molten metal filters of the prior art, a number of problems still exist. A typical thermite coating system consists of five key ingredients namely: Fe.sub.2 O.sub.3, alloys of Fe/Al or Cr/Al, graphite, organic binder, and solvent. The presence of a toxic and flammable solvent in this system poses both health and environmental hazards, particularly in a production environment involving a large scale operation utilizing a substantial amount of highly volatile solvents. Cost is also a concern due to the use of relatively expensive organic binders and solvents and the need to safely dispose of the spent solvents.
Accordingly, it is a principal objective of the present invention to provide an improved thermite coated molten metal filter, more specifically, an aqueous thermite coated molten metal filter, particularly suitable for the filtration of molten iron without the problems discussed above.